The invention relates to a method for controlling an electric pump drive motor of a power steering device, in which a high output stage and a low output stage are provided, in which the motor can be operated, a switchover between the two output stages taking place as a function of the current consumption of the motor.
From EP 0 673 328 B2, and corresponding U.S. Pat. No. 5,761,627, both of which are incorporated by reference herein, a power-assisted steering system is used in association with a not-shown mechanical steering system in order to reduce the steering torque to be applied via the steering wheel. This is realized by means of a hydraulic cylinder 1, the piston rod 2 of which is connected to a steering tie rod 3 (illustrated in FIG. 3). The chambers 4 and 5 of the hydraulic cylinder 1 are charged with hydraulic fluid in dependence on the steering deflection by means of a pump 7 via a servo-valve 6, through the lines 8 and 9.
The hydraulic diagram in FIG. 1 shows that the hydraulic fluid is pumped from a reservoir 10 to the hydraulic cylinder 1 via the servo-valve 6 by means of the pump 7. If the hydraulic fluid is, for example, pumped into the chamber 4 via the line 8, the piston 11 that is arranged on a piston rod 2 is moved toward the left such that the hydraulic fluid is displaced from the chamber 5 back to the servo-valve 6 via the line 9. This hydraulic fluid then flows back into the reservoir 10. In the neutral position of the hydraulic cylinder 1 and the servo-valve 6 which is illustrated in FIG. 1, no hydraulic fluid reaches the hydraulic cylinder 1. The hydraulic fluid is only pumped from the reservoir 10 back into the reservoir 10 through the servo-valve 6 by means of the pump 7. A pressure control valve 12 that produces a short circuit between the pump 7 and the reservoir 10 for instances in which the hydraulic fluid cannot be discharged via the servo-valve 6 or the servo-valve 6 and the hydraulic cylinder 1 is also arranged in the hydraulic diagram for safety reasons.
The pump 7 that serves for conveying the hydraulic fluid is driven by an electric motor 13 that is supplied with voltage by the battery 14 of the motor vehicle via electronics 21. A detailed illustration of the electronics 21 is shown in FIG. 2. In order to realize a voltage supply of the electric motor 13 that depends on the respective requirements, a current sensor 15 is provided which measures the power requirement of the pump 7 and conveys said power requirement to a switching transistor 16 via an electronic control unit 17.
Instead of the current sensor 15 illustrated in FIG. 2. the power-assisted steering system according to FIG. 3 can also be equipped with a proximity sensor 18 that scans the position of a suitable structural component in the steering gear 19. The information of the proximity sensor 18 is again fed to the electronic control unit 17 which evaluates this signal and determines the supply voltage of the electric motor 18 therefrom.
Also from EP 0 673 328 B2, and corresponding U.S. Pat. No. 5,761,627, a method is known for operating the system shown in FIG. 1 in which the hydraulic pump 7 for the conveying of hydraulic fluid to the servo valve 6 can be driven by the electric motor 13 in two defined output stages, a standby operation and a full-load operation. As illustrated in the diagram of FIG. 4, the motor 13 is operated in the full-load operation A with a high speed n, so that the pump generates a high hydraulic flow Φ. In standby operation B, the motor speed n is reduced to such an extent that only a small hydraulic circulation is maintained. With an increased power requirement of the pump 7, the motor 13 is switched into full-load operation A, or vice versa back into standby operation B again. As a measurement for the output requirement of the hydraulic pump, the current consumption of the motor is detected by means of the current sensor 15. The current sensor is connected with the control unit 17 which evaluates the values of the current sensor and if necessary carries out a switchover from one output stage of the motor into the other.
The two output stages A, B, as can be seen from the diagram of FIG. 4, are independent of the velocity v of the vehicle. An adaptation of the output stages A, B to the vehicle velocity v, i.e. a lowering of the motor speed n and hence of the hydraulic flow Φ on increasing the vehicle velocity is dispensed with, because with such a lowering in case of need (e.g. in demanding situations with regard to travel dynamics with sudden, rapid steering maneuvers) sufficient volume flow can not be made available in order to provide the necessary steering assistance.